In the drilling and completion of an oil or gas well, a cementing composition is often introduced in the well bore for cementing pipe string or casing. In this process, known as ‘primary cementing’, a cement slurry is prepared at the surface and is pumped into the annular space between the casing and the walls of the well bore. The cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath. This isolates the well bore into subterranean zones to prevent the undesirable migration of fluids between zones.
There are two main factors that contribute to ensuring zonal isolation during the life of a well. Specifically, the cement should be placed in the entire annulus through efficient mud removal and the properties of the set cement should be optimized so that it can withstand the stresses from various operations that may be conducted during the life of the well.
If the short-term properties of the cementing composition, such as density, static gel strength, and rheology are sufficient, the undesirable migration of fluids between zones is prevented immediately after primary cementing.
Poor zonal isolation results in fluid migration e.g. water or gas may invade an oil-bearing zone, which can eventually result in a risk of blow out, or to less severe but economically challenging problems such as water production (and the need to provide expensive water treatment surface facilities) or loss of reserves and productions. Remedial work to repair a faulty cementing job is expensive (inasmuch as it increases rig time and delays oil or gas production) and can sometime lead to irreparable harm to the hydrocarbon-bearing production. In most cases, poor zonal isolation results from poor mud removal as the well is initially filled with mud.
It is desirable to adjust the density of the cement such that the pressure exerted by the cement on the formation at the bottom of the well compensates at least for the formation pore fluid pressure so as to avoid any risk of influxes. The upper limit of hydrostatic pressure generated by the column of cement plus the head losses due to the circulation of the fluids being pumped must remain below the fracturing pressure of the rocks in the section being cemented. Certain geological formations are very fragile and require densities lower to that of water to avoid such fracturing.
Since the risk of influx diminishes with column height, the density required for compensating pore pressure can be lower if a large column height is used. Furthermore, cementing over a large height is advantageous since that makes it possible to reduce the number of sections that must be cemented. All of these factors favor the use of cement slurries of low density.
To produce lighter (less dense) slurries, the simplest known technique is to produce extended slurry in which the quantity of water is increased compared to normal slurry while adding stabilizing additives (known as “extenders”) to the slurry for the purpose of avoiding settling of particulate materials and/or formation of free water at the surface of the slurry.
Among the most common extenders that are used are sodium silicate and bentonite. One of the main features of these extended slurries is their very low plastic viscosity that is a consequence of the low solid volume fraction (less than 45%). This very low viscosity can very often be the main cause of poor mud displacement and therefore of a bad zonal isolation.
The simplest known technique to increase the plastic viscosity is to add a high molecular weight polymer to the slurry (as cellulosic based polymers). These polymers are generally available in a solid form. This can be an issue from a logistical point of view, specifically for offshore applications when liquid additives are preferred.
These polymers are also often available in oil-based emulsion, but this tends to result in products that are very expensive products and/or have a bad environmental footprint.
Therefore an object of the invention is to provide a water based thickening additive.